If you mean a gradual lift to the point (the blade looks to be gently "smiling" when looking at it edge-on) then it's intentional, and known as a "crown". Not all American blades come with a crown to them but it's generally looked at as a positive. The stroke with an American scythe tends to be ergonomically improved by a gentle crowning since the swing of the arms naturally wants to operate in a plane that's at an angle vs. the plane of the ground. The crown keeps the region of the blade doing the cutting more in line with the ground at any given part of the stroke without as much deliberate thought needing to be put into it. A slight side-to-side "scoop" to the cut is used with a crowned blade and once you get used to it it's much more comfortable for extended mowing sessions than one without. An uncrowned blade may have one introduced through gentle hammering of the chine (the raised lip of the spine) along its length on a good anvil.
Got any pictures of the unit you picked up? I'd be happy to tell you what I can about it.
Also, you may have seen this before, but it bears reposting:
This is the present version of the guide I've written on American scythes. In examination of the technique employed I think you'll find that while the primary engine of the stroke does, indeed, come from the upper body as opposed to tension in the hips/waist/lower back, the entire body is still employed under most (though not all) normal mowing circumstances.